1. Field of the Invention
This invention relates to a pneumatic radial tire, and more particularly to a heavy duty pneumatic radial tire for use in so-called heavy vehicles such as middle-size or more truck and bus, industrial vehicle, construction vehicle and the like. Particularly, the invention relates to a pneumatic radial tire having a long service life and excellent bead portion durability without arranging an extra reinforcing member in a bard portion or a region from a bead portion to a sidewall portion and increasing the volume of the rubber filler in the bead portion.
2. Description of Related Art
In a radial tire running under a heavy load, the sidewall portion is largely bent, but also the bead portion existing outward from a flange of a rim in a radial direction of the tire falls down toward the outside of the tire (hereinafter referred to as tire outside simply) or causes so-called bead portion fall-down phenomenon. By such a phenomenon a large compression strain.multidot.stress is applied to a turnup end portion of a carcass existing in the bead portion or in the sidewall portion near to the bead portion.
Further, a large movement of members is produced in a region ranging from the bead portion to the sidewall portion located in correspondence to stepping-in zone and kicking-out zone in a ground contact area of a tread portion of the radial tire along peripheries of these portions. Also, a large torsional deformation is created in these portions when a certain slip angle is applied to the tire during the cornering of the vehicle and hence a large shearing strain stress based on these member movement and torsional deformation is applied to the turnup end portion.
Since the above large compression strain stress and shearing strain stress are repeatedly input to the turnup end portion during the running of the tire, fatigue of rubber proceeds and finally rubber cracking is created along the turnup end portion, which is liable to advance to a separation failure of the turnup end portion. Such a failure is a time-honored and new problem with respect to the pneumatic radial tire running under a heavy load. This new problem is brought up by a high requirement of weight reduction and cost reduction and a tendency of a low section profile having, for example, an aspect ratio of not more than 70%.
In order to solve the above separation failure, various countermeasures have been taken. Among them, a main part in a typical embodiment of the conventional countermeasure is shown in FIG. 7, in which a rubber filler 7 taperingly extending from a bead core 4 toward an end of a tread portion (not shown) is arranged between a main body 5a of a carcass and a turnup portion 5b thereof along an outer surface of the main body. It is divided into a hard rubber stock 7-1 located side the main body 5a and a soft rubber stock 7-3 located side the turnup portion 5b.
When the solution is not yet satisfied by the illustrated embodiment, it is attempted to take various countermeasures for the distribution of the hard rubber stock 7-1 and soft rubber stock 7-3. As shown in FIG. 7, for example, the section shape of the hard rubber stock 7-1 is thickened in the vicinity of a position opposite to an end of the turnup portion 5b, or the hard rubber stock 7-1 is selectively arranged in the vicinity of this position.
Although it is expected to considerably improve the separation failure of the end portion of the turnup 5b to thereby attain remarkable improvement of bead portion durability, it has actually been confirmed that satisfactory bead portion durability cannot be obtained by these countermeasures. A peeling failure between cord and coating rubber in the main body 5a of the carcass is caused as a negative compensation for the improvement of separation resistance in the end portion of the turnup 5b and such a peeling failure concentrates in a region located somewhat outward from a position opposite to the end of the turnup portion 5b in the radial direction of the tire. This failure is a quite inexperienced and novel form of trouble.